Hi All - My battery is not getting charging voltage from the alternator. I removed the alternator and took it to a shop. The mechanic said that a diode in the regulator is blown and that he is unable to find the diode in his catalog. He said he might have more luck if he had some identification of the alternator. Can anyone identify this alternator, off a late model Moyer rebuild from 2009?

The boat had a starting battery and a house battery, but the house battery faulted and I removed it altogether.

The diode may have blown from being routed to the faulted battery before I removed it, but looking at the wiring diagram it appears to me that the alternator output goes to an isolator and then is routed to both batteries without going through the A-B switch. A friend with much more electrical ability than me checked the resistance through the isolator and said he thought it was defective. He suggested that I remove the isolator and tie the three red wires to the isolator together.

If I can get the alternator repaired I would like to. Or, buy a new one from the Moyer site, if it will drop in place of the old one. Or, beg, borrow or steal a small gas generator to keep the battery charged enough to get me to the Pamlico Sound where there are Atomic 4 mechanics who can fix my charging system. I have not been able to find anyone to do the work at my present location on the upper Albemarle Sound.

In 2009 we were still using rebuilt Motorola alternators that were standard on all late model Atomic 4s from the 1970s. The only history we have as to the cause of output diode failures is from running the alternator to an open circuit (like with the battery selector in OFF). As far as I know, the output diodes were pretty standard on all that vintage of Motorola, so I'm a bit surprised that a local starter/alternator shop was not able to match them up for you and install a new set. The only advise I can think of - other than to replace the alternator - is to try and find another local automotive starter/alternator shop for a second opinion, or go back to the one you already went to and try to tactically find another person to talk to. You may have gotten a counter person that was having a bad day and you simply didn't have his/her full attention. Meanwhile, I'm hoping that some senior Forum member with a better handle on electrical issues than I will come to your rescue with some solution that I'm simply not aware of. Don

Thanks, Mr. Moyer, that may help us out a lot. The mechanic is a good and knowledgeable man but he's distracted by some serious health issues that his wife is having, and anything I can do to make his work easier will help. Knowing that it is a Motorola unit might trigger his memory. He says he has a source in the midwest that sometimes has parts for vintage alternators and maybe they can supply the diode.

Prestolite Leece-Neville bought out Motorola’s line of alternators a few years ago. Yours is probably a 37 amp model MR12N451D. The Prestolite web site is not easy to navigate, but the parts list for this alternator can be found at:

From your description, it's not clear to me whether the alternator or the regulator has failed. The purpose of the regulator is to send current to the field windings in the alternator. If the regulator has failed (no current to field windings), you can bypass it and send current to the field through a toggle switch. In essence, YOU become the regulator - turn on field current when battery voltage gets low, turn it off before you cook your batteries.
One of the early external regulators was the AutoMac (sold by Weems & Plath) which I used for years. It had two controls - an ON/OFF switch and a current adjustment. It would charge at the current you selected until the battery came up to voltage, then shut itself off. In use, I would then dial back the current a bit and restart the regulator.

Thanks for the responses. With that parts number, the mechanic has found a replacement for me. In his opinion, the cost of the diodes and labor would be close enough to the cost of a new alternator to make it better to just replace the whole thing. So he is ordering one and will have it for me next week. He will show me how to hook it up, in comparison to the old one.

Since I removed the isolator on suspicion that it was not working correctly, I have three red wires to tie together - a positive from the house battery, a positive from the starting battery, and a positive leading to the alternator. Can I put in a toggle switch with the two positives from the battery on one side, and the positive to the alternator on the other, to make a field disconnect switch, so I can turn the current to the alternator off when the engine is not running?

No. Most toggle switches are rated at no more than 10 amps. If you mean one wire is from the alternator output (you said “to the alternator”) then that can carry, and switch, the full alternator output. With your Motorola alternator that is 37A. A 10A toggle switch might last as long as several milliseconds before its contacts melted or vaporized. If you get a higher output alternator, you will need a switch that can safely handle switching the higher output, something like,

Tac, I have just one red wire on my alternator, attached next to the + sign as shown in the photo I posted of the alt. I am proposing to put a toggle rated for 20 amps at 125 volts which I think should be safe for around 200 amps at 12 volts where the isolator used to be, with the lead between the alternator and the toggle on one side and the two leads to the battery positives on the other. What do you think? Would that be a usable plan? My concern is that with the direct connection between the batteries and the alternator, I could see battery voltage at the alternator all the time, even with the engine shut down and the A-B switch in the off position.

In short, an alternator’s output is controlled by regulating (changing) the current to the field windings. This is done two ways:

1. A “regulator” is included as part of the alternator. This is what you have. The regulator senses the battery voltage and uses a small(ish) current (0-5A or so) to the alternator’s field windings. The lower the field current, the lower the output current from the alternator (stator current). The alternator’s output, the stator current, is 3 phase AC, and is then converted to DC inside the alternator, generally using a bridge rectifier (6 or more diodes - “rectifier” is merely a 4 syllable word for the 2 syllable “diode”). In your case, the regulator is behind the finned retangular metal piece in your first photo. The two #16 (or so) wires coming out of that finned box (one is/was white, the other is now copper) are connected to the alternator’s field winding via carbon brushes (P/N 14) to a set of internal slip rings (P/N 18), at that nut.

2. The alternator has no built in regulator. You buy a regulator and connect its output to the alternator’s field winding terminals. The external regulator senses battery voltage and adjusts the field current, 0-5A, to control the alternator output.

In your case, the Motorola alternator output is a #8 (red) wire to the battery(s) + terminals through the ammeter and something called a “Volt Div”, which I guess is someone’s attempt at describing an Isolator, and a battery selector switch. A #8 wire is rated, for an engine space, at 68 amps. This is sufficient for the 37A alternator.

What Al was describing was eliminating any alternator internal regulator, and controlling the alternator output directly from the battery using a switch. In the control system world this is called bang-bang control, since the alternator output is switched on and off, rather than smoothly controlled over the full field 0-5 amp range. This used to be the standard way on cars, before about 1970, to regulate alternator output, using a voltage sensitive relay (regulator) on the firewall which contained a set of contact points that would switch the field current on and off. The John Luckless regulators on English cars were notorious for these contacts burning out every few thousand miles.

If you want to convert an internally regulated alternator to an externally regulated one, it ain’t difficult - see:

Switch ratings are NOT linearly scaleable. A switch rated at 20A @ 12VDC is NOT therefore rated at 200A at 120VDC. If the manufacturer does not specify, the rating for a 20A 12VDC switch would be 2A at 120VDC. It’s related to the power the switch can handle (volts x amps). To complicate it even more, a switch rated at 12VDC is often rated at a lower current at 12VAC. Likewise, your switch rated at 20 amps at 125 volts (probably AC) is highly unlikely to be rated at 200A DC. I’d be surprised if it were even rated as high as 20ADC. In short, always check the manufacturer’s data sheet, or ask them.

Your plan is to switch the alternator’s output off with a manual switch on the #8 red wire. That will certainly disconnect the alternator from the battery, but when you switch the alternator’s output off with the engine running, the output at the alternator stator momentarily goes VERY high, generally high enough to destroy one or more diodes in the bridge rectifier, which reduces the alternator’s output to an .....ummm......unusable level. If you are correct that the “isolator” is defunk-it, then if it internally failed open circuit, it probably caused the alternator’s rectifiers to fail (go BOOM!).

Your concern about having battery voltage at the alternator output when the engine is off is no problem. That’s how the great majority of cars and boats are wired. The diodes in the bridge rectifier (if good) will only see battery voltage, less than 14VDC, or so. These diodes usually have a reverse voltage rating of over 100 volts, and so isolate the battery from the alternator, until the alternator’s output exceeds the battery voltage (i.e. the engine is running). Moverover, many alternators include an “isolation” or “blocking” diode right after the bridge rectifier. It’s purpose is to further isolate the alternator from the battery. If I ‘member right, the Motorola has this diode (P/N 11).

Thanks, Tac - I can see that the alternator output - the red positive wire - goes through an ammeter to what this schematic calls "volt div" which I agree must be the isolator - then from there to the batteries. So I can dispense with the "volt div" and tie those three red wires together and the alternator will send current to the two batteries?

"Your concern about having battery voltage at the alternator output when the engine is off is no problem" - OK, that's good, I don't have any need to put a toggle switch, which I have seen called a field disconnect switch, anywhere in the circuit.

One thing that is confusing me is looking at the schematic and it looks like the Selector Switch is not between the alternator and the batteries. It looks like the only thing the selector switch does is control whether one or the other or neither or both batteries supply voltage to the starter. But I won't worry about it unless I need to.

So my plan will be to install the new alternator and wire it just like the old one, but dispense with the isolator, tying the three red wires together. Then I should be able to set the Selector Switch to the fully-charged starting battery, start the engine and put a multimeter across the battery terminals and see the alternator output.

If I'm missing something or getting ready to harm the electrical system, please let me know. If I can get this boat back to the Pamlico, I'll have a competent mechanic go over the engine and get it in shape. As you can see, mechanical/electrical is not my forte. Thanks again for your patient help.

I did post the schematic at the beginning of the thread. The only thing I'm proposing to change is, the device labelled "Volt Div", that has one wire coming from the alternator and two wires going out to the batteries, to tie all three wires together and delete the "Volt Div" which Tac and I both think is the defective isolator.

I think you understand this, but let me emphasize it. Your setup, with the Motorola alternator, or any alternator with a built-in regulator, does NOT allow you to connect wires to the alternator field. There are no terminals for you to connect to the field winding. That is because the alternator’s internal regulator connects there internally. So, don’t even think about connecting from the battery to the alternator field via a switch, or anything else.

For example, pg 32 shows some of their manual battery switches. One of these is probably what your “sketch” calls “Sel Sw”. When properly wired, it allows you to select which battery, #1 or #2, is connected to the house/engine loads and alternator (like the 6008), or OFF. Roadnsky’s post shows such a setup.

Your switch may have a ”Both”, or “1&2”, position (6007), allowing both batteries to be connected together. This, from your description, appears to be what you’re aiming for.

All these are manual switches, and are generally used to switch between two similar batteries on a daily rotation, to give equal usage and charge.

If you connect the 3 wires going to the “Volt Div” together, the alternator output will always go to both batteries. While that will work, you won’t be able to disconnect the alternator and house loads from the batteries when in the OFF position. This is a safety thing - in event of a fire or emergency, you’d like to put the switch in the OFF position and disconnect everything from the batteries and alternator. The alternator output and the house loads should be connected to the selector switch’s COMMON terminal (which may or may not be the unnumbered terminal in your sketch), as shown on pg 32.

On Pg 40, Automatic Charging Relay (ACR) and Isolators, is shown how an ACR is connected to do this switching automatically, and how an Isolator is hooked up. From your sketch, you appear to have an Isolator (dead or alive).

The catalog appendix on pg 160, Battery Management Schematics for Typical Applications, shows some common connections with an ACR. Yours may be the one labeled “2 Battery - 1 Engine”. (As part of a marketing philosophy of “don’t confuse the customer, he’ll buy more”, they don’t show an alternator. Instead you get an engine).

Thanks, Tac, that is a great help and I am starting to understand. One thing that gave me problems was I saw in Don Casey's book "This Old Boat" on pages 236 and 237 a description of a field disconnect switch and it included a sketch that showed a switch in what he labeled "supply wire" which ran from the battery + to the regulator. That's why I thought I could put a switch where the isolator used to be.

But I think what you are telling me is to run the big + cable out of the alternator/regulator to the common at the selector switch and run the battery + cables to the 1 and 2 connectors. What I still wonder about, is the sketch that I posted earlier, that came from the previous owner of the boat, shows that the common already has a cable from it to the starter. So is it ok to stack the cable from the alternator on that one?

Here is my sketch of what I currently understand that I should do. I have omitted the wires that I wouldn't change from the current setup, like the one from the alternator to the coil, and the ground, which I understand isn't really a wire, just a representation of the alternator being grounded to the engine by being bolted in place. Does this look like a workable arrangement?

I had problems using the alternator foot for grounding. The problem was identified when I checked the voltage between the block and the alternator case with the alternator charging. I added a separate ground wire from the case to the block. You may not need it, but check.
Most alternators (the Motorola included) are meant for automotive use. Their regulators (internal or attached) have a control voltage that I find to be too low for effective battery charging.
The alternator itself is not a complex device. The windings that make the power are located in the case, don't rotate, and have their AC output turned into DC by diodes pressed into the case. The rotor carries the field winding, which gets its current through two brushes and slip rings.
The regulator acts to control this field current. Two types of regulators: First is P type which controls the current going TO one brush, the other brush is grounded. Second is N type which controls the current coming FROM one brush, the other brush is attached to the alternator output. The alternator has to be wired to match the regulator and vice versa.
Second suggestion is to get rid of the A/B switch and use two separate battery banks, each controlled by a simple On/Off switch. The start battery (I found no need for anything larger than a Group 24) is connected to the starter and engine loads. House bank is connected to your distribution panel, and can be configured to your liking with more/less bigger/smaller batteries.
Batteries are charged from the alternator. The system that worked best for me was to send the alternator output to the house bank. When the house bank came up to voltage, a relay (VCR) would close and send current to the engine battery.
There are lots of ways to skin a cat. I've tried several, and this is what works for me. Hope it helps.

Well, Yes and/or (inclusive OR) No. I’ll be cautious. I have no idea what this “Sel Sw” is. What are it’s contacts rated for? A picture (Suitable Fo’ Framin’) of the front of this switch, and the back of the switch, is necessary. For example, as connected, it appears that a terminal (possible the Common or Output, who knows, it’s not labeled) is connected to the starter solenoid. The starter solenoid is mounted on the starter and draws only about 12A. The solenoid, when energized by the start switch, switches a large current (about 125A normally. But if the engine is frozen, or the propeller jammed, it can draw a hellacious amount more) from the battery(s) to the starter motor during the start. There is no wire size shown for this starter motor wire (supply red, return is through the engine block). It should be BIG, like #4 or #2.

As to “stacking” wire terminals, others may correct me, but I believe the ABYC and Coast Guard limit is 4 connections max on any terminal stud or screw. And that’s physically pushing it. You can get around this by using power posts (see the Blue Sea catalog pg. 102), appropriately sized for current and wire terminal size.

Ideally, the starter motor (as well as the solenoid, through the start switch) should be connected to the selector switch Common/Output along with the alternator and house loads, as roadnsky shows. This means it is Imperative that the selector switch be able to handle well over 200A. The Blue Sea selectors are designed for over 450A.

Thanks for hanging in there with me. I'm about 240 miles from the boat, I go down there about once a month for a few days and work on it and then come home - but I did dig up a picture with the front of the switch in it:

The back of the switch I'll have to wait until next time I'm at the marina to photograph. I'm not too concerned about the switch being inadequate, the boat only has 150 amps of batteries provided for and the previous owner was a very by-the-books type who had the boat maintained by Bock Marine in NC. The issues I am having I attribute to the boat resting at the dock almost unused for about 8 years before I bought it. I think the owner enjoyed having all the bells and whistles and gradually complexified the electrical and other systems to a point that they are hard to understand for a novice. I have focused on chainplates, standing rigging, spreaders, deck leaks and so on and am just now starting to give the engine and electrical system some attention. Up here on the Albemarle there are no serious boatyards or technicians who I could turn to on these issues so I'm trying to resolve them to the extent that I can sail the boat back to the Pamlico where there are several good yards. Sorry to be so long-winded.

Your isolator and alternator failure may have been caused by opening (disconnecting) the alternator output while the engine was running. This causes a momentary (microseconds long) spike which results from the conversion of energy stored in the stator windings to voltage (Faraday’s Law). You will notice on the front of your disconnect/selector switch it says, “BEFORE SWITCHING TO OFF POSITION STOP ENGINE”. Well, that’s why.

But wait! There’s more!

Many “ordinary” rotary switches, and some old battery selector switches, have what are called “Break-Before-Make” contacts. That is, if the switch is in position 1 and its contacts are closed, when you turn it to position 2 the contacts first open (Break) before they reach the 2 position, where they then close (Make). As you can see, if you are in position 1 on battery #1, and the engine is running, when you switch to position 2 on battery #2, no matter how blinding-fast you are, the contacts will open, the alternator output is opened, and POP! goes another diode (or was that a rectifier?).

So most of these battery selector switches today are “Make-Before-Break”. That is, on switching from position 1 to 2, the contacts remain closed to position 1 until the switch is in position 2. Then, with the switch contacts closed in position 2, the contacts in position 1 can now open. Voila! The alternator output is never opened, and All's Well.

Well, almost all. You can still kill a running alternator by switching to the OFF position. Stop the engine first. Then switch to OFF.

This one goes Off>1>Both>2. Yes, I was severely warned when I bought the boat never to turn it to off with the engine running, but it's possible I did it inadvertently, though I doubt it. The switch itself feels a little loose as it goes from position to position, so maybe it is losing contact between settings. As you said, it's probably at least 20 years old. Anyway, my policy will be to set it to both and never change it until the engine is off, at least until I can confirm the switch is ok or replace it.

I’ve taken your sketch of post 1 and revised it in the first diagram below to show the changes needed to remove the Isolator. I have not shown any fuses other than what is shown on the original. Some comments:

1. Your sketch shows #8 wires connecting the battery negatives. This is too small. You say in post 19 that you have 150A of batteries. Since batteries are not rated in amps, but in amp-hours (Ah), I assume you mean 150Ah. Would that be two 75Ah batteries?
2. The proper size black cable connecting their negatives might be a #0 (1/0), not a #8. Likewise, the wires going to the starter motor, to handle the 125A current, should be at least #2, not #8. The wire to the Ignition and Start switches and then to the starter solenoid, drawing a mere 12A, can be a #10 or #12, depending on the total length.
3. USCG and ABYC require fuzes within 7” of each battery. If you don’t have them, I would recommend the MRBF fuses, shown in the Blue Sea catalog on pg 64. Although somewhat pricey, they’re easy to install, and take up little room.
4. You may end up with more than 4 wires on the output terminal of the battery selector switch. If so, add a power post, like in the Blue Sea catalog on pg 102.
5. Your original sketch shows a selector switch, “SS Sel Sw”, connected by #10 wires to the “Volt Div” isolator. With the house loads now connected to the battery selector switch output, this 2 position SS Sel Sw is no longer needed. I’ve shown this on the lower diagram.

This is not meant to be a course in basic electricity. If you will be doing much of the work (and planning) yourself I recommend the following as minimum reading. It will also serve to check the work and advice of others (especially if the advice is free, like this, but also if you’re paying for it):

Thanks, Tac - Yes, two 75 ah batteries. That sketch with post 1 isn't really MY sketch, it's one that I found in the papers aboard the boat when I bought it. I don't think those cables are actually #8s, don't know why the sketch has them labeled like that.

The only sketch I take credit for is the rather crude one in post, I believe, 16. After long perusal I am just starting to get some small understanding of the first sketch, and I will study the ones you made for me and hopefully come to understand them. Many thanks for taking the time to make them.

This is getting too deep for me when all I want is to get from the Albemarle Sound to the Pamlico Sound, where I can find a competent mechanic. I'd sail the boat the whole way, except there are two canals enroute, one of them 20 miles long, and I'm going to have to motor through them. I need enough battery power to start the engine a few times and run it for about 8 hours.

So I'm going to wire it up like in my crude sketch, with a couple of marina friends watching me and giving me free advice. I'm going to beg, borrow or steal a small gas generator for backup. And then I'm heading for the Pamlico.

Thanks All for your advice, and thanks especially Tac for taking the time and effort to try to bring me along.

1. Your proposed (simplified) hookup in post #16 looks OK.
2. The black and red cables to the batteries look like #2s.
3. The battery connections, especially the lower battery’s + lead, are badly corroded. If this is the Start battery, and you want to have a good starting engine, this connection must be seen to. Corrosion means resistance, which means voltage drop. This leads to reduced current to the starter, and slow (or no) starts.
4. For a temporary reliable, safe system, those connections need, at a minimum, cleaning up. Replacing the cables would be better. You could kill 2 birds with one stone (fuse protection, a USCG requirement, and good connection) and use the Blue Sea 5191 or 2151 terminal fuse blocks for the batteries.
5. The lower battery red cable, with the clamp-on terminal, is heavily corroded. I betcha it has corrosion underneath the jacket for some distance up the cable. If so, it needs replacement.
6. What is that tank in the lower right corner, with what looks like a Moeller 3 Way Fuel Valve? I hope it’s a water tank. If it is a fuel tank, it needs IMMEDIATE attention! The two green hoses do not look like USCG approved, ethanol rated, fuel hose (i.e. Type A1-15, SAE J1527). Besides being illegal, use of non-approved hose can, especially in the presence of ethanol fuel, lead to dissolved hose innards clogging the carburetor. Murphy’s Law dictates that this must happen in restricted waters, while dodging a 120,000 DWT tanker. The other clear, reinforced hose heading off to the left is designed for water, and has no place in a fuel system.